A nitride semiconductor has been considered to be applied for a high withstand voltage and high-power semiconductor device by using characteristics such as high saturation electron velocity and wide band gap. For example, a band gap of GaN being the nitride semiconductor is 3.4 eV, and it is larger than a band gap of Si (1.1 eV) and a band gap of GaAs (1.4 eV), and has high breakdown electric field intensity. Accordingly, GaN is extremely expectable as a material of a semiconductor device for a power supply in high voltage operation and obtaining high-power.
As a device using the nitride semiconductor, a lot of reports have been made as for a field effect transistor, in particular, a high electron mobility transistor (HEMT). For example, in a GaN-based HEMT (GaN-HEMT), an AlGaN/GaN.HEMT in which GaN is used as an electron transit layer and AlGaN is used as an electron supply layer attracts attention. In the AlGaN/GaN.HEMT, a distortion resulting from a difference in lattice constants between GaN and AlGaN is generated at AlGaN. High-concentration two-dimensional electron gas (2DEG) is obtained by a piezoelectric polarization generated thereby and a spontaneous polarization of AlGaN. Therefore, it is expected as a high withstand electric power device such as a high-efficiency switch element and an electric vehicle.    [Patent Literature 1] Japanese Laid-open Patent Publication No. 2009-289827    [Patent Literature 2] Japanese Laid-open Patent Publication No. 2005-243727
In a nitride semiconductor device, an art locally controlling a generation amount of the 2DEG is required. For example, in case of the HEMT, it is desired that a current does not flow when a voltage is turned off, so-called a normally-off operation from so-called a fail-safe point of view. A device is necessary to suppress the generation amount of the 2DEG at downward of a gate electrode when the voltage is turned off to enable the above.
As one of methods enabling a GaN.HEMT performing the normally-off operation, a method is proposed in which a p-type GaN layer is formed on an electron supply layer, the 2DEG existing at a portion corresponding to beneath the p-type GaN layer is ceased to be directed to the normally-off operation. In this method, p-type GaN is grown at a whole surface of, for example, on AlGaN to be the electron supply layer, the p-type GaN is dry-etched to remain at a formation portion of the gate electrode to form a p-type GaN layer, and the gate electrode is formed thereon.
As stated above, the dry-etching is used for a patterning of the p-type GaN. A surface layer of the electron supply layer disposed under the p-type GaN is damaged by the dry-etching, as a result, a sheet resistance (Rsh) and a contact resistance (ρc) increase, and an on-resistance decrease. In this case, it is impossible to obtain an enough on-current (drain current) even though a gate voltage is applied. In addition, there is a problem in which a large variation occurs at the drain current.